6S2 A MANUAL OF PHYSIOLOGY 



nearest the median fissure, and those from the higher roots farthest 

 away from it. Other collaterals from the posterior root-fibres, and 

 many of the root-fibres themselves, run into the anterior horn; some 

 pass into the posterior horn, and doubtless come into relation with 

 its scattered cells and with the cells of Clarke's column. Other 

 collaterals and probably some axons cross the middle line in the 

 anterbr and posterior commissures and end in the grey matter of the 

 opposite side. 



We may, therefore, conclude without hesitation that some 

 of the fibres of the posterior roots ascend to the medulla in 

 the posterior column of the cord without forming synapses 

 with any cells until they reach the gracile and cuneate nuclei, 

 where they end by breaking up into terminal brushes of 

 fibrils. The cell-bodies of these neuroris lie in the posterior 

 root-ganglia. 



Connections of the Direct or Dorsal Cerebellar Tract. Since 

 the dorsal or direct cerebellar tract does not degenerate after 

 section of the posterior nerve-roots, but does degenerate 

 above the level of the lesion after section of the spinal cord, 

 the nerve-cells from which its axons arise must be situated 

 somewhere or other in the cord. Now, it has been observed 

 that the vesicular column of Clarke first becomes prominent 

 in the lower dorsal region, and that in this same region the 

 direct cerebellar tract begins. Atrophy of the cells of Clarke's 

 column has sometimes in disease been shown to accompany 

 degeneration of the direct cerebellar fibres. After an experi- 

 mental lesion of these fibres in animals, some of the cells of 

 the vesicular column show the changes in the Nissl bodies 

 which we have already described, under the name of chrom- 

 atolysis, as occurring in nerve-cells whose axons have been 

 cut. After two or three months these cells may be found 

 almost completely atrophied (Schafer). Finally axis-cylinder 

 processes have been seen sweeping out from Clarke's column 

 into the direct cerebellar tract (Mott). The evidence, then, 

 is very strong that some of the cells of Clarke's column are 

 the cells of origin of this tract. Clarke's cells are surrounded 

 by arborisations, some of which represent the terminations 

 of posterior root-fibres and of their collaterals. .The direct 

 cerebellar tract runs right up to the cerebellum through the 

 restiform body, without crossing and without being further 

 interrupted by nerve- cells. The restiform body ends partly 



